Secure async servlet redispatches

[Sequela02]

Summary

• preserve authenticated SecurityContext across ASYNC and ERROR redispatches with RequestAttributeSecurityContextRepository
• cover DeferredResult, emitters, streaming, statelessness, authorization, concurrency, Tomcat SSE disconnects, and committed responses
• document official contracts, alternatives, migration, compatibility, and prepare 7.2.0

Verification

• ./gradlew.bat clean check --no-daemon
• ./gradlew.bat qualityCheck build publishToMavenLocal --no-daemon
• embedded Tomcat RST test repeated successfully